Salting-out effect of alkali metal chlorides (lithium, sodium, and potassium) on 2-cyanoguanidine aqueous solution: A solid–liquid equilibrium study

Abstract

Solubility of 2-cyanoguanidine in three alkali metal chlorides aqueous solution (LiCl, NaCl and KCl) was measured by visual polythermal method at temperature range from 298.15 K to 328.15 K. The experimental values show that solubility of 2-cyanoguanidine decreased with the concentrations of alkali metal chlorides increasing at a given temperature and salt concentration from wt = 0.01 to 0.05. Solubility data of 2-cyanoguanidine in KCl aqueous solution is significantly higher than that in NaCl or LiCl aqueous solutions. Each set of experimental measured values were correlated using the modified Apelblat equation as a function of temperature. It's obvious that modified Apelblat model shows good agreements with experimental solubility data for three systems. Simultaneously, the salting-out yields (S–O) of three alkali metal salts (LiCl, NaCl, and KCl) on the solubility of 2-cyanoguanidine in water were computed, among all the temperatures and three alkali metal chlorides aqueous solutions, which increase from (0.0407, 0.0089, and 0.0087) to (0.5234, 0.4669 and 0.3561), respectively. Also, salting-out behaviors of three alkali metal chlorides were investigated by Setschenow equation, which shows the relative mole fraction solubility of 2-cyanoguanidine decreased in the following order: LiCl > NaCl > KCl. This result keeps high consistency with the variational tendency of the solubility data. The dissolution enthalpy dH° (27.8–38.6 kJ mol−1), dissolution entropy dS° (52.3–87.6 J mol−1 K−1) and molar Gibbs energy dG° (9.5–13.2 kJ mol−1) of 2-cyanoguanidine in three chlorides aqueous solutions calculated on the basis of modified Apelblat equation analysis are all positive, which suggests that dissolution of 2-cyanoguanidine in alkali metal chloride aqueous solutions was endothermic, spontaneous, and entropy-driven process.